Sealing window units better against water leakage, particularly in driving rainstorm conditions, has long been a goal of window manufacturers. In fact, in order to earn the Hallmark certification of the Window and Door Manufacturers Association (WDMA) for water penetration resistance (and other certifications), a window must pass a rigorous test such as tests conducted under the ASTM E547-00 and ASTM E331-00 test methods. The test generally seeks to model a severe rainstorm with wind driven rain and may require a window to be exposed, for instance, to a pressure drop (based on 15% of required wind resistance design pressure) with water sprayed at a rate of 5 gallons per square foot per hour. This rate of water theoretically creates a film of water on all surfaces of the window to assure that the water bridges any breaches in the weatherstrip seals around the window sashes. If a leak through these seals occurs, water can be propelled through the breach due to the lower pressure on the interior side of the window, which results in a water penetration failure.
Double hung windows are especially hard to seal against water penetration in a driving rain because there are two parallel vertically sliding sashes in different planes. To accomplish a seal, horizontal compression seals generally are applied along the top rail of the upper sash and the bottom rail of the lower sash to seal against the header and sill respectively of the window. Single vertical sliding compression seals generally are applied between the interior edges of the stiles of the upper sash and the jambs, while single vertical sliding exterior compression seals generally are applied between the exterior edges of the stiles of the lower sash and the jambs. A horizontal compression checkrail seal resides between the check rails of the upper and lower sashes when the sashes are closed. Finally, a horizontal sliding seal bridges the upper and lower sash vertical sliding compression seals and the end of the checkrail seal. The complexity introduced by changes in plane and functionality of sliding sashes in double hung windows creates barriers to the success of developing a watertight single barrier seal. While the upper sash is often the easiest to seal successfully, it customarily is more difficult to create reliable seals at the transition from the upper sash seals to the lower sash seals and at the bottom corners of the lower sash seals. As a consequence, some leakage of water at these and perhaps other locations is inevitable, particularly in a blowing rainstorm.
Some window manufacturers have addressed water leakage past the seals of a double hung window by catching water that does leak in a reservoir. Many times, the reservoir is formed by an interior sill stop that projects upwardly from the sill along its interior edge and overlies some of the interior face of the lower sash bottom rail. The resulting reservoir will catch and contain a given volume of water. However, if the volume of water that leaks through the seals exceeds the volume of the reservoir, which can happen in prolonged storms, the reservoir can overflow the sill stop resulting in an unacceptable interior leak. Another technique is to catch water that has leaked through the seals in a reservoir and purposefully drain the water back out through a defined path such as a weep hole. This technique requires a reservoir with a water column height great enough to ensure that the water pressure in the reservoir overcomes the exterior pressure in a blowing rainstorm. Water is then able to flow back out to the higher pressure outdoor or exterior side of the window. This drains water from the reservoir and prevents water from overflowing into the interior of the building.
An improved water management system for double hung windows is needed and it is to the provision of such a system that the present disclosure is primarily directed.